Double acting servomotor



July 9, 1935.

F. W. DAVIS DOUBLE ACTING -SERVOMOTOR Filed April 10,'1933 'w/zZ-LW ywww en@ u r y f "1 ma@ m1.

Patented July 9, 1935 y UNITED STATES DOUBLE ACTING SERVOMOTOR FrancisW. Davis, Belmont, Mass.

Application April 10, 1933, Serial No. 665,329

6 Claims.

This invention relates to a servo-motor having an element movable in onedirection for a power stroke and in the opposite direction for a returnstroke. A motor of this type is particularly adapted and intended foruse in applying brakes on a vehicle, but it is evident that it iscapable of other equivalent uses especially where a strong pull in onedirection only is desired. The particular embodiment of the inventionhereinafter shown and described is particularly adapted for use onautomobiles and for operation by means of a liquid such as. oil or aglycerine-water mixture, supplied under pressure by a suitable pump.Most liquids undergo an increase in viscosity at lower temperatures,this being true of most oils. Hence, when oil is employed in theoperation of a servo-motor such as is described in my Patent No.1,846,089 the thickness and viscosity of the oil when it is cold mayprevent a sufficiently' invention reference may be had to thedescription thereof which follows and to the drawing of which l Figure 1is a longitudinal sectional view of a servo-motor embodying theinvention. Figure 2 is a sectional view of the forward en ofthe motor,taken on the line 2-2 of Figure 1. Figure 3 is a fragmentary'sectionalView of the rear end of the apparatus shown in Figure 1.

Figure 4 is a diagrammatic representation of several partsof anautomobile, showing how the invention may be employed. y

' Figures 5 and 6 are fragmentary portions of Figure 1, showing theparts in different relative positions of operation. 1

Referring to'Figure 4, the motor I0 for applying the brakes may beconveniently used in conjunction with a power-actuated steering For almore complete understanding of this houses the steering gear-|| theremayalso be a' from the outlet of the steering gear to the inlet 20 ofthe servo-motor for the brakes. The discharge outlet 2| of this motor isconnected as by a return pipe 22 to a reservoir 25 from which the inletof the pump I2 is supplied as by a pipe 26. While any suitable uid.,preferably a liquid, may be employed in the operation of the presentinvention, oil is a convenient liquid for use in an automobile or thelike. For convenience, the power fluid is hereinafter referred to asoil. The invention, as shown, is embodied in a casing 30 `having aforwardly projecting extension 3| of reduced diameter, terminating in anear 32 by which the motor may be secured to the automobile frame orother suitable support.. The casing 30 encloses a main chamber 35 havinga forward extension 3B within the reduced portion 3| of the casing.AReciprocable in the casing 3|] is a piston 40 having a rearwardlyextending piston rod 4| projecting through the rear end of the easingand suitably packed as at 42. Atv the rear end of the rod 4| is a yoke43 to which may be secured a brake rod 44 adapted to apply one or morebrakes 45 by a tension on the rod 44. The piston rod 4| is extendedforwardlyof the piston 4U in the form of a stem 46 which is adapted totelescope with the casing extension 3|, being slidably fitted therein.The piston rodv4| and stem 46 constitute al continuous holldw tubewithin which isslidably tted a valve member 5I). The forward portion ofthe valve member is hollow as at5|, the rear end Aof the valve memberprojecting from the 'rear ,end of the piston rod 4I and terminating in-ayoke 52 which is connected to the brake pedal lever 53 of theautomobile. Rearward movement of the valve member 50 is limited byengagement of a shoulder 54 with a cap 55 mounted on the rear end of thepiston rod 4|, the yoke 43 vof the piston rod. being shown as integralwith the cap 55. The valve member 50 is normally held in its extremerearward position as by a suitable spring 5T,l which is compressedbetween a'shoulder `in the for-ward end-.of the valve member 5|) and ashoulder within a thimble 58 screwed into "the open' end of the stem 46.The stem 46 vis provided lwith an annular groove 60 communicating withIthrough the wall of the valve member 50.

the interior of the stem through a series of holes 6|. The groove 60opens into the chamber 35 when the piston is in its extreme rearwardposition as shown in Figure 1. Thus oil entering the inlet 20 normallyflows through thev chamber 36 into the interior of the stem 46, throughthe holes 6| and the groove 60 into the main chamber 35, and thenceoutwardly through the outlet 2|. Since these passages offer a relativelyunrestricted ilow for the oil through the casing, there is but littleoil pressure ordinarily in the casing. It may be noted that suchpressure as there may be at times in the chamber 36 acts against thevalve member 50, tending to assist the spring 51 in pressing the valvemember rearwardly. Oil pressure in the chamber 36 also acts against theforward end of the stem 46, the effective area of pressure being equalto the annular area of cross 'section of this stem. For the operation ofthe motor, the valve element 50 is provided with a pair of externalannular grooves 65 and 66, these grooves being separated by an annularrib 61. The groove 65 is in constant communication with the interiorhollow by a series of holes 66 Opposite this rib 61 is an annular groove10 in the inner wall of the piston rod 4| and stem 46, the groovecommunicating through a series of holes 1| with a pressure chamber 15 inthe casing 30, rearward of the piston 40. The groove 66 communicateswith the cha'mber 35 forward of the piston as by a number of holes 16.As shown in Figure 1, the groove 65 in the valve is normally cut offfrom communication with the groove 10 but can be moved intocommunication therewith by forward movement of the valve'member 50relative to the piston 40, that is, by taking up a portion of thelost-motion between these members. The rib 61 is not as wide as thegroove 10 so that when a portion of the lost-motion between the valvemember and piston has been taken up, the groove 65, and hence the hollow5|, is connected not only with the chamber 15 rearward of the'piston,but also through the groove 66 and the holes 16 with the chamber 35"forward of the piston. Hence, oil entering the ,holes 16 into thechamber 35. Thus more or less pressure is built up in the chamberaccording to the relative size of the clearance on either side of therib 61. As soon as there is any forward movement on the part of thepiston 40, the groove 60 is closed by the wall of the casing extension3| so that direct communication between the inlet and outlet throughthis groove is thereafter cut o until the piston has resumed its normal,fully retracted position. groove 60 has been closed by a small forwardmovement of the piston, or by suiicient relative movement by the valvemember 5|) to-take up all the lost-motion between the valve member andthe piston, whereby the forward end face 80 of the valve member 50engages.. an annular rib 8| within the stem 46, all the oil enteringAthe chamber 36 must flow through the holes 1| and 16 if onlya portionof the lost-motion has been ward movement of th valve member 50sufficient When the to take up all the lost-motion between it and thepiston 40 directs the entire flow of the oil entering the inlet into thepressure chamber 15 where the pressure acts against the rear face of thepiston 40, this rear face having an area of considerable magnitudecompared with the end area of the stem 46 and the cross-sectional areaof the valve 50 against which the oil presses in the opposite direction.As long as the lost motion between the valve member 50 and the piston4|! is completely taken up, the full pressure of the oil is exertedagainst the rear face of the piston, thus maintaining a maximum forwarddrag on the brake rod 44. When pressure on the brake pedal lever 53 isremoved, the spring 51 together with oil pressure acting rearwardlyagainst the Valve 50, pushes the valve member 50 rearwardly so thatfurther flow through the holes 66 and the groove 65 is completely cutoff. The oil pressure in the chamber 15 is relieved thorugh the holes1-|, the groove 66, and the holes 16, into the chamber 35 and outthrough the discharge outlet 2|. Maximum oil pressure however, is stillpresent in the chamber 36 and is exertedagainst the forward end of thestem 46 and against the valve member 50, this pressure tending to moveboth members rearwardly. Thus there is a positive rapid return movementIof the brake pedal when the pedal has been released by the operator,this return movement being caused by pressure in the chamber 36. As thecross sectional area of this chamber is small compared to the pressurearea on the piston 40, the return movement is more rapid than theworking movement for a given rate of flow of oil.I As soon as the piston46 and valve member 50 have reached their extreme retracted positions,the grooves 60 opens into the chamber 35 and again provides for a directflow from the inlet 20 'to the outlet 2|. In order to limit the maximumbraking force to be applied by means of the motor, a suitable by-pass 65is provided between the inlet 20 and the outlet 2|, a a spring loadedrelief valve 66 being connected in this by-pass to relieve any excesspressures above a predetermined maximum which may occur in the inlet 20.The valve 66, as shown, is pressed against its ,seat as by a spring 61,the physical characteristics of which determine' the maximum pressure'.A ball check 66 may be provided within the valve 66, this check beingheld on its seat as by a light spring 69. Thus, if pressure in thedischarge 2| should exceed pressure in the inlet 20, as when the brakesare applied when no power fluid is being supplied, this excess in thedischarge 2| is at once relieved past the check valve 66,.

'I'he piston 46 is also provided with a check valve in the form of anannular plate 60 closing a circular series of holes 9| of suitable sizesufficient to permit a relatively large flow of oil from the chamberv 35into the chamber 15 when the piston is moved forwardly. The plate 66 isresiliently held in place as by a number of spring washers 62 on screws83. Thus if the pump I2 is being driven slowly so that there isinsufficient flow 'of oil to ll the chamber 15 as rapidly as it expandswith a quick forward movement of the piston 40, suction in the chamber15 which might result from such rapid forward movement of the pistonisfeasily relieved by ilow of oil through the openings 9| from thechamber 63.,

Inthe upper portion of thecasing 30 is an relief passage 95 opening intothe chamber 1l 35 when the piston is in its fully retracted posi.

tion. Thus when oil under pressure is rst directed into the chamber anybubble which may have formed at the uppermost end thereof is quicklydischarged through the vent ,95 and is swept into the discharge 2|, thevent 95 soon thereafter being closed by forward movement of the piston.

From the foregoing description of certain embodiments of this invention,it should be evident to those skilled in the art that various changesand modifications might be made therein without departing from itsspirit or scope as defined by the appended claims.

I claim:

1. Mechanism of the class described, comprising a casing having a mainchamber with a reduced extension at the forward end thereof, a pistonreciprocable in said main chamber forwardly on a working stroke andrearwardly on a return stroke, said piston having a hollow stemextending forwardly andslidably fitted in said chamber extension, aymanually operable valve element within said stem with a lost-motionconnection thereto, a fluid supply inlet atthe forward end of saidchamber extension, and a fiuid discharge outlet at the forward end ofsaid main chamber, said piston and element having valve openingscooperating to afford free passage of power fluid from the inlet to theoutlet only when the piston and element are both at their extremerearward position, and to direct power fiuid against the rear face ofthe piston when' said 10stmotion is taken up.

2,. Mechanism of the class described, comprising a casing having a mainchamber with a reduced extension at its forward end, a piston slidablein said main chamber and having a hollow stem slidably fitted in saidextension, and a manually operable element slidably fitted within saidstem, said piston and element normally being in their extreme rearwardposition, a fluid inlet at the forward end of said extension, a fluidoutlet at the forward end of said main chamber, said stem havingopenings through its wall normally providing direct passage for fiuidfrom said extension to the forward portion of said main chamber, saidpiston and element also having openings cooperating upon take-up of saidlostmotion to direct fluid against the rear face of said piston.

3. Mechanism of the class described, comprising a power-actuated memberhaving a forward working stroke from a normal position of rest and arearward return stroke, said member having front and rear opposed facesof substantially equal area and an additional front face of lesser area,all said faces being adapted to be pressed by power uid employed for theactuation of the mechanism, means including a manually operable controlelement having a lost-motion connection with said member for balancingfluid pressure against said opposed faces when said element ls in normalposition relative to said member and the member is in its normalposition and for directing power fluid against the additional front faceonly when the element is in its normal position relative to said memberbut the member is in a forward position.

4. Mechanism of the class described, comprising a casing with an inletand an outlet, a reciprocable fluid power actuable member slidable insaid casing from and to a normal position of rest, a control elementhaving a lost-motion connection with said member and cooperating there-`with to direct power fluid against surfaces of said member to producefollow-up movement, said member having a passage connecting said inletand outlet when the member is in its normal position and adapted to beclosed by said casing whenever said member is displaced from its normalposition.

5. Mechanism of the class described, comprising a casing having a main'chamber therein and a forwardchamber of lesser diameter opening intosaid. main chamber, an inlet duct opening into the forward end of saidforward chamber, an outlet duct opening into the forward end of saidmain chamber, a piston slidably fitted in said main chamber and providedwith an extension lslidably fitted in said forward chamber, and acontrol element movable with said piston and capable of limited movementrelative thereto to direct power fluid entering said inlet against therear face of said piston.

6. Mechanism of the class described, comprising a casing having a mainchamber and a forward chamber of lesser diameter opening thereinto, aninlet duct opening into the forward end of said forward chamber, anoutlet duct opening into the forward end of said main chamber, a pistonslidably fitted in said main chamber, said piston having a hollowopen-ended extension slidably fitted in said forward chamber, saidextension having a passage through its wall opening into said mainchamber when the piston is in its rearmost position and cut off from themain chamber when the piston and extension move forwardly, and a valveele-ment having a lostmotion connection with said piston and adapted tocooperate therewith to direct power fluid from said inlet duct againstthe rear face of said piston.

FRANCIS W. DAVIS.

